Log pinpoints timing of Puget Sound earthquake 1,100 years ago

A Douglas fir log plucked from a sewer trench along the shores of Puget Sound has helped scientists narrow the time frame for a major earthquake more than a millenium ago, the last big rupture of the Seattle fault.

The earthquake had been dated to about A.D. 900 to 1000. However, new radiocarbon measurements narrow the time to between 900 and 930, Brian Atwater, a University of Washington affiliate professor of geological sciences and U.S. Geological Survey geologist, said today (May 3) during a presentation at the Seismological Society of America’s annual meeting in Seattle.

The size of the log isn’t certain, since part of it remains buried. The exposed portion is about 5 feet long and 2.5 feet in diameter. It probably rode a tsunami generated by an earthquake on the Seattle fault, Atwater said. The earthquake caused a bulrush marsh at West Point, on the Puget Sound shore in Seattle, to drop and the tsunami deposited sand on top of the marsh. The log, discovered when the West Point sewage treatment plant underwent major expansion in the early 1990s, flattened bulrush stems as it came to rest on the sand.

Evidence shows the log hadn’t been dead long, he said. It clearly hadn’t been rotting in a forest or rolling on a beach, because when it was discovered in 1992 bark still covered all but its top side.

Radiocarbon dating and examination of the log’s outer 15 rings indicated an age similar to that for bulrush stems in the subsided marsh. The ages were determined by Minze Stuiver, now an emeritus geological sciences professor who operated the UW radiocarbon dating laboratory before it closed last year. Atwater used other data from Stuiver to convert the radiocarbon figures to calendar years. That provided a time range of A.D. 900 to 930 for the death of the tree from which the log came.

The high precision of Stuiver’s radiocarbon ages is helping scientists to understand earthquake hazards from the Seattle fault, Atwater said.

“The earthquake on the Seattle fault about 1,100 years ago made plenty of geologic history, leaving an uplifted terrace on Bainbridge Island and Alki Point, sunken forests in Lake Washington and the sand sheet at West Point,” he said.

The research is a small part of a long-term effort to define earthquake hazards in the Puget Sound region. As the hazards become better defined, public options for mitigating the hazards also might become clearer, Atwater said.

“To understand big earthquakes on the Seattle fault, it is important to know whether they coincide with earthquakes on other faults,” he said.

That includes the boundary between the Juan de Fuca and North America tectonic plates off the coast of Washington, Oregon and British Columbia, which has been the source of great subduction earthquakes in Cascadia. Narrowing the date of the last big Seattle fault earthquake to A.D. 900 to 930 will mean more precise determination of whether Seattle fault earthquakes coincide with those on other faults, once dating of similar precision is obtained for earthquakes on other faults. Dating work already is under way for faults beneath southern Puget Sound and along the Pacific coast.

Atwater said the range of A.D. 900 to 930 also probably applies to both the bulrush marsh subsidence and tsunami at West Point and to a complementary rise in the terrain along the south side of the Seattle fault, which includes the Alki Point area of West Seattle and the south end of Bainbridge Island. It also applies to strong shaking in what is now Seattle if, as tree-ring patterns indicate, the dated log died during the same year as trees that were carried by landslides to the bottom of Lake Washington.